The present invention relates to a method of making radioactive metal articles having small dimensions, such as wire, foil, and rods. In particular, the present invention concerns a method in which the concentration of radioactive species in the wire or other article is not limited by the concentrations of such species or their precursors naturally occurring in the metal from which the wire is made.
Certain medical treatments make use of radioactive materials in the form of wire or filaments. For example, radiation therapy for cancer patients sometimes makes use of radioactive wires that can be implanted or inserted in the patient's body to provide a therapeutic dose of radiation to an area of the body, i.e., to destroy tumor cells. For example, the radioactive wire can be implanted into a solid tumor in the patient's body, or inserted into a blood vessel, where it can be left for a time sufficient to provide the desired dose of radiation to the intended site in the patient's body.
Another medical procedure that could make use of radioactive wire is the prevention of restenosis (gradual reclosure) in coronary arteries after angioplasty. It is believed that application of radiation to a region of a coronary artery that has been dilatated by angioplasty can inhibit restenosis, by inhibiting cell growth in that region.
The Ir-192 isotope, with a half-life of 79 days, is one material that is currently used in the medical treatment of certain cancers. In one method now in use, short lengths of natural iridium wire are irradiated to produce Ir-192, by neutron capture of Ir-191. The iridium wire segments are sealed within a cavity at the end of a larger diameter wire of another material, typically a nickel-titanium alloy. The larger diameter wire can then be implanted or inserted into the target area of the patient's body.
However, the abundance of Ir-191 in natural iridium is only 37%. This limits the concentration of Ir-192 that can be generated by irradiation of standard iridium wire. There is a need for more intense radiation sources for therapy, in order to provide higher doses of radiation to a site in a patient's body, or to provide a given dosage of radiation in a shorter time. In some cases there is a need for smaller diameter radiation sources of the same intensity, in order to further localize the region of tissue affected during a radiation treatment.
One possibility for producing iridium wire comprising a higher concentration of Ir-192 is to form the wire in the first instance from iridium that has already been enriched in Ir-191 compared to natural iridium, followed by irradiation of the wire to convert the Ir-191 to Ir-192. However, this has not been practical in the past because of the high cost of the enriched iridium (presently about $10,000/g) and the limited quantities available (typically less than 100 g). Although there are a limited number of commercial producers of natural iridium wire, there is no previously existing practical method of fabricating wire starting with such limited quantities of material.
A need exists for improved methods of making radioactive wire that can produce a wire having an enhanced concentration of radioactive species from very small amounts of starting materials.